Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 230759 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 359 AM EDT Sun Jul 23 2017 .SYNOPSIS... High pressure remains off the southeast coast. A weak boundary stretches from central Virginia westward to low pressure over southern Illinois. An upper level trough of low pressure will move over the northeastern US early in the week. && .NEAR TERM /TODAY/... Another day, another challenging forecast... Surface analysis reveals a weak frontal boundary extending from the northern neck of Virginia...across southern WV back to southern IL. This has provided the trigger for multiple mid summer MCS formation from VA back to MO. This can be seen clearly on IR imagery. While the storms of Saturday brought the Mid Atlantic a cooler overnight the dewpoints remain in the low to mid 70s, and 88D STP product estimates 2+" rainfall totals over the Highlands. While the beam is scanning higher there, and thoughts of ice contanimation enter the thought process a check of river/stream gages shows rapid rises (now cresting/crested and well below flood stage). But this area will need to be monitored today if more heavy rain falls. WHile convection moving into southern WV seems like it would be on a track to potentially impact the southern fringes of our forecast area the latest HRRR shows the cells tracking NE and weakening during the morning hours. Models then seem to be in agreement that more convection will develop this afternoon, possibly focused over PA. Bottom line is there will be rainfall across much of the forecast area today - question is where and how much. For now am holding off on any thoughts of a flood watch. Dayshift will no doubt reevaluate. Next concern is severe potential. Model soundings are showing CAPE in excess of 1000 this afternoon. Low level wind fields are weak. The amount of CAPE we receive will be dependent upon the amount of breaks in cloud/sunshine/heating the region sees. As discussed above cloud cover could make heavy rainfall the greater threat. And there is still the heat threat. Although not as hot as during the second half of the week temperatures still reach the lower 90s...possibly mid 90s south of DC. This combined with the extremely moist air mass will cause heat indices to exceed 100 in the I-95 and east area. Question becomes "will these reach 105?" We will ponder this further in the next hour or two. && .SHORT TERM /TONIGHT THROUGH TUESDAY/... SREF implies that the 00-06Z timeframe could be busy as far as heavy rainfall, but it is also hinting that it will be maily north of our foreast area. Am carrying likely PoPs across much of the area this evening. We will remain in this 70+ dewpoint air mass overnight. Lows therefore will again exceed 70 east of the mountains. The heat wave busting short wave will be tracking into the northeastern US Monday, but relief will not be arriving until Monday night and Tuesday. Highs Monday again in the low 90s with the chance for isolated convection. Tuesday`s temperatures will generally top out in the mid to upper 80s. Dewpoints will fall back into the 60s. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... Ridge of high pressure will be centered over New England Tuesday night delivering lower humidity and noticeably cooler weather on low level northeasterly flow. Lows in the 60s to near 70F. The high will then quickly shift offshore by Wednesday, with winds shifting from the northeast to southeast. There could be some scattered showers/thunderstorms south/west nearest to the old frontal boundary and across the higher terrain. High temperatures likely to be a touch below normal with highs generally in the low to mid 80s. By Thursday/Friday, the flow will turn around to the southwest ahead of an approaching frontal system. Temperatures/moisture will likely spike back up ahead of the front, and along with that the chances for showers/thunderstorms. Some uncertainty exists with regards to the progressiveness of the front, and this will play into the forecast for Saturday. If front slows or stalls, chances for showers/thunderstorms may persist into the weekend. Highs Thursday/Friday back up to near 90F. && .AVIATION /07Z SUNDAY THROUGH THURSDAY/... VFR conditions will prevail through Monday...outside of thunderstorm activity. Thunderstorms are possible this afternoon and evening with gusty winds and reduced visibility in the strongest storms. VFR Tuesday. Predominantly VFR expected Tuesday night through Thursday. However there may be some patchy fog/low clouds both Wednesday and Thursday mornings with low level marine flow. Chances for showers/thunderstorms also increase on Thursday. && .MARINE... Winds will remain below SCA values through Monday. Thunderstorms capable of producing gusty winds above Special Marine Warning criteria are possible today. No marine hazards Monday night or Tuesday. Mainly sub-SCA winds expected Tuesday night through Wednesday, generally northeasterly Tuesday night, turning southeasterly by Wednesday. Winds may then approach SCA criteria Wednesday night into Thursday as they increase out of the south/southwest. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ PRODUCTS...WOODY!/MJM

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